1. Field of the Invention
This invention relates to a field-effect transistor formed in a nitride based compound semiconductor and, in particular, to a field-effect transistor that can be designed to be significantly low in source and drain parasitic resistance values, very high in drain saturation current, and very high in transconductance. Also, this invention relates to a fabrication method for the field-effect transistor.
2. Description of the Related Art
Wide bandgap nitride based compound semiconductors such as GaN, AlGaN and GaInN attract attention as a material for the field-effect transistor including a high-voltage and high-output HEMT (high-electron-mobility transistor).
Semiconductor devices generally include an ohmic electrode and a non-ohmic electrode.
In general, when a field-effect transistor is fabricated by using a wide bandgap system semiconductor crystal, for example, by forming on a sapphire substrate an undoped GaN layer as an electron transport layer (or channel layer) and a high-resistivity or n-type AlGaN as an electron supply layer on which a source electrode and a drain electrode are formed, the series resistance of these ohmic electrodes will increase to reduce the performance.
JP-A-10-223653 discloses a recess type field-effect transistor that a high-concentration region is formed by ion implantation of Si etc. as well as an electronic device formation epitaxial crystal growth method etc., and that an electrode is formed on the high-concentration region to reduce the series resistance of the ohmic electrode.
However, since the recess type field-effect transistor needs the formation of a non-ohmic electrode, it is necessary to remove a part of the high-concentration region by etching etc. in order to secure a region where to form the electrode. This may cause increased unevenness on the surface of the field-effect transistor as well as difficulty in the fabrication process, which are significantly disadvantageous in wiring an electrode etc.